Method for reducing melt fracture during extrusion of ethylene polymers

ABSTRACT

A method for reducing the melt fracture during extrusion of a molten narrow molecular weight distribution ethylene polymer which comprises extruding said polymer through a die having a die gap greater than about 50 mils and wherein at least a portion of one surface of the die lip and/or die land in contact with the molten polymer is at an angle of divergence or convergence relative to the axis of flow of the molten polymer through the die.

FIELD OF THE INVENTION

This invention relates to a method for reducing melt fracture,particularly sharkskin melt fracture, during extrusion of a moltennarrow molecular weight distribution ethylene polymer, under conditionsof flow rate and melt temperature which would otherwise produce suchmelt fracture, which method comprises extruding said polymer through adie having a die gap greater than about 50 mils and wherein at least aportion of one surface of the die lip and/or die land in contact withthe molten polymer is at an angle of divergence or convergence, relativeto the axis of flow of the molten polymer through the die.

BACKGROUND OF THE INVENTION

Most commercial low density polyethylenes are polymerized in heavywalled autoclaves or tubular reactors at pressures as high as 50,000 psiand temperatures up to 300° C. The molecular structure of high pressurelow density polyethylene is highly complex. The permutations in thearrangement of its simple building blocks are essentially infinite. Highpressure resins are characterized by an intricate long chain branchedmolecular architecture. These long chain branches have a dramatic effecton the melt rheology of the resins. High pressure low densitypolyethylene resins also possess a spectrum of short chain branchesgenerally 1 to 6 carbon atoms in length which control resincrystallinity (density). The frequency distribution of these short chainbranches is such that, on the average, most chains possess the sameaverage number of branches. The short chain branching distributioncharacterizing high pressure low density polyethylene can be considerednarrow.

Low density polyethylene can exhibit a multitude of properties. It isflexible and has a good balance of mechanical properties such as tensilestrength, impact resistance, burst strength, and tear strength. Inaddition, it retains its strength down to relatively low temperatures.Certain resins do not embrittle at temperatures as low as -70° C. Lowdensity polyethylene has good chemical resistance, and it is relativelyinert to acids, alkalis, and inorganic solutions. It is, however,sensitive to hydrocarbons, halogenated hydrocarbons, and to oils andgreases. Low density polyethylene has excellent dielectric strength.

More than 50% of all low density polyethylene is processed into film.This film is primarily utilized in packaging applications such as formeat, produce, frozen food, ice bags, boilable pouches, textile andpaper products, rack merchandise, industrial liners, shipping sacks,pallet stretch and shrink wrap. Large quantities of wide heavy gage filmare used in construction and agriculture.

Most low density polyethylene film is produced by the tubular blown filmextrusion process. Products range from 2 in. diameter or smaller tubesof film to be used as sleeves or pouches, to huge bubbles that provide alay flat of 20 ft. (when slit along an edge and opened up will measure40 ft. wide).

Polyethylene can also be produced at low to medium pressures bypolymerizing ethylene or copolymerizing ethylene with variousalpha-olefins using heterogeneous catalysts based on transition metalcompounds of variable valence. These resins generally possess little, ifany, long chain branching and the only branching to speak of is shortchain branching. Branch length is controlled by comonomer type. Branchfrequency is controlled by the concentration of comonomer(s) used duringcopolymerization. Branch frequency distribution is influenced by thenature of the transition metal catalyst used during the copolymerizationprocess. The short chain branching distribution characterizingtransition metal catalyzed low density polyethylene can be very broad.

U.S. patent application Ser. No. 892,325 filed Mar. 3, 1978, and refiledas Ser. No. 014,414 on Feb. 27, 1979 in the names of F. J. Karol et aland entitled Preparation of Ethylene Copolymers In Fluid Bed Reactor,discloses that ethylene copolymers, having a density of 0.91 to 0.96, amelt flow ratio of ≧22 to ≦32 and a relatively low residual catalystcontent can be produced in granular form, at relatively highproductivities if the monomer(s) are polymerized in a gas phase processwith a specific high activity Mg-Ti containing complex catalyst which isblended with an inert carrier material.

U.S. patent application Ser. No. 892,322 filed Mar. 31, 1978, andrefiled as Ser. No. 012,720 on Feb. 16, 1979 in the names of G. L. Goekeet al and entitled Impregnated Polymerization Catalyst, Process ForPreparing, and Use For Ethylene Copolymerization discloses that ethylenecopolymers, having a density of 0.91 to 0.96, a melt flow ratio of ≧22to ≦32 and a relatively low residual catalyst content can be produced ingranular form, at relatively high productivities, if the monomer(s) arepolymerized in a gas phase process with a specific high-activityMg-Ti-containing complex catalyst which is impregnated in a porous inertcarrier material.

U.S. patent application Ser. No. 892,037 filed Mar. 31, 1978 and refiledas Ser. No. 014,412 on Feb. 27, 1979 in the names of B. E. Wagner et aland entitled Polymerization Catalyst, Process for Preparing And Use ForEthylene Homopolymerization discloses that ethylene homopolymers havinga density of about ≧0.958 to ≦0.972 and a melt flow ratio of about ≧22to about ≦32 which have a relatively low residual catalyst residue canbe produced at relatively high productivities for commercial purposes bya low pressure gas phase process if the ethylene is homopolymerized inthe presence of a high-activity Mg-Ti-containing complex catalyst whichis blended with an inert carrier material. The granular polymers thusproduced are useful for a variety of end-use applications.

The polymers as produced, for example, by the processes of saidapplications using the Mg-Ti containing complex catalyst possess anarrow molecular weight distribution, Mw/Mn, of about ≧2.7 to ≦3.6, andpreferably, of about ≧2.8 to ≦3.4.

LOW DENSITY POLYETHYLENE:RHEOLOGY

The rheology of polymeric materials depends to a large extent onmolecular weight and molecular weight distribution.

In film extrusion, two aspects of rheological behavior are important:shear and extension. Within a film extruder and extrusion die, apolymeric melt undergoes severe shearing deformation. As the extrusionscrew pumps the melt to, and through, the film die, the melt experiencesa wide range of shear rates. Most film extrusion processes are thoughtto expose the melt to shear at rates in the 100-5000 sec⁻¹ range.Polymeric melts are known to exhibit what is commonly termed shearthinning behavior, i.e., non-Newtonian flow behavior. As shear rate isincreased, viscosity (the ratio of shear stress, τ, to shear rate, γ)decreases. The degree of viscosity decrease depends upon the molecularweight, its distribution, and molecular configuration, i.e., long chainbranching of the polymeric material. Short chain branching has littleeffect on shear viscosity. In general high pressure low densitypolyethylenes have a broad molecular weight distribution and showenhanced shear thinning behavior in the shear rate range common to filmextrusion. Narrow molecular weight distribution resins of the presentinvention exhibit reduced shear thinning behavior at extrusion gradeshear rates. The consequences of these differences are that the presentnarrow distribution resins require higher power and develop higherpressures during extrusion than the high pressure low densitypolyethylene resins of broad molecular weight distribution and ofequivalent average molecular weight.

The rheology of polymeric materials is customarily studied in sheardeformation. In simple shear the velocity gradient of the deformingresin is perpendicular to the flow direction. The mode of deformation isexperimentally convenient but does not convey the essential informationfor understanding material response in film fabrication processes. Asone can define a shear viscosity in terms of shear stress and shearrate, i.e.:

ζ shear=τ12/γ

where

ζ shear=shear viscosity (poise)

τ12=shear stress (dynes/cm²)

γ=shear rate (sec⁻¹) an extensional viscosity can be defined in terms ofnormal stress and strain rate, i.e.,:

ζ ext=π/ε

ζext=extensional viscosity (poise)

π=normal stress (dynes/cm²)

ε=strain rate (sec⁻¹)

Due to the high shear stress developed during extrusion of a highmolecular weight ethylene polymer having a narrow molecular weightdistribution melt fracture, particularly sharkskin melt fracture,occurs. Sharkskin melt fracture has been described in the literature fora number of polymers. "Sharkskin" is a term used to describe aparticular type of surface irregularity which occurs during extrusion ofsome thermoplastic materials under certain conditions. It ischaracterized by a series of ridges perpendicular to the flow directionand is described by J. A. Brydson Flow Properties of Polymer Melts, VanNostrand-Reinhold Company (1970), pages 78-81.

In the present process, the onset of sharkskin melt fracture isdetermined by visual observation of the surface of an extrudate formedwithout take-off tension from a capillary die. Specifically thisprocedure for determining sharkskin melt fracture is as follows: a 40×magnification microscope is used. The extrudate is lighted from theside. The microscope shows the transition from a low-shear, glossysurface of the extrudate to a critical-shear, matted surface (the onsetof sharskin melt fracture) to a high-shear, deep ridge, sharkskin meltfracture. This method is generally reproducible to ±10 percent in shearstress.

The narrow molecular weight distribution ethylene polymers as describedherein exhibit the characteristics of sharkskin melt fracture uponextruding. These characteristics include a pattern of wave distortionperpendicular to the flow direction; occurrence at low extrusion rates(less than expected for elastic turbulance); no effect of common diematerial; and less melt fracture with increasing temperature.

There are several known methods for eliminating sharkskin melt fracturein polymers. These methods include increasing the resin temperature.However, in film formation this method is not useful since increasingresin temperature generally causes lower rates, due to bubbleinstability or heat transfer limitations. Another method for eliminatingsharkskin is described in U.S. Pat. No. 3,920,782. In this methodsharkskin formed during extrusion of polymeric materials is controlledor eliminated by cooling an outer layer of the material so that itemerges from the die with a reduced temperature while maintaining thebulk of the melt at the optimum working temperature. However, thismethod is difficult to employ and control.

In the present method melt fracture, particularly sharkskin meltfracture, can be virtually eliminated by geometric changes in the die,i.e., by extruding the narrow molecular weight distribution ethylenepolymer, at normal film extrusion temperatures, through a die having adie gap greater than about 50 mils and wherein at least a portion of onesurface of the die lip and/or die land in contact with the moltenpolymer is at an angle of divergence or convergence relative to the axisof flow of the molten polymer through the die. The method of thisinvention is only possible due to the observation that the stress fieldat the exit of the die determines the creation of sharkskin meltfracture. This is, sharkskin melt fracture can be controlled oreliminated by the geometry at the exit of the die and is independent ofdie entrance or die land conditions.

Films suitable for packaging applications must possess a balance of keyproperties for broad end-use utility and wide commercial acceptance.These properties include film optical quality, for example, haze, gloss,and see-through characteristics. Mechanical strength properties such aspuncture resistance, tensile strength, impact strength, stiffness, andtear resistance are important. Vapor transmission and gas permeabilitycharacteristics are important considerations in perishable goodspackaging. Performance in film converting and packaging equipment isinfluenced by film properties such as coefficient of friction, blocking,heat sealability and flex resistance. High pressure low densitypolyethylene has a wide range of utility such as in food packaging andnon-food packaging applications. Bags commonly produced from low densitypolyethylene include shipping sacks, textile bags, laundry and drycleaning bags and trash bags. Low density polyethylene film can be usedas drum liners for a number of liquid and solid chemicals and asprotective wrap inside wooden crates. Low density polyethylene film canbe used in a variety of agricultural and horticultural applications suchas protecting plants and crops, as mulching, for storing of fruits andvegetables. Additionally, low density polyethylene film can be used inbuilding applications such as a moisture or moisture vapor barrier.Further, low density polyethylene film can be coated and printed for usein newspapers, books, etc.

Possessing a unique combination of the aforedescribed properties, highpressure low density polyethylene is the most important of thethermoplastic packaging films. It accounts for about 50% of the totalusage of such films in packaging. Films made from the polymers of thepresent invention preferably the ethylene hydrocarbon copolymers, offeran improved combination of end-use properties and are especially suitedfor many of the applications already served by high pressure low densitypolyethylene.

An improvement in any one of the properties of a film such aselimination or reduction of melt fracture or an improvement in theextrusion characteristics of the resin or an improvement in the filmextrusion process itself is of the utmost importance regarding theacceptance of the film as a substitute for high pressure low densitypolyethylene in many end use applications.

DRAWINGS

FIG. 1 shows a cross section of die;

FIG. 2 shows a cross section of a spiral die;

FIG. 3 shows various configurations of die gaps;

FIG. 4 shows a fluid bed reactor in which the ethylene polymers may beprepared.

SUMMARY OF THE INVENTION

It has now been found that melt fracture, particularly sharkskin meltfracture formed during extrusion of a molten narrow molecular weightdistribution ethylene polymer, can be reduced by extruding said polymerthrough a die having a die gap greater than about 50 mils and wherein atleast a portion of one surface of the die lip and/or die land in contactwith the molten polymer is at an angle of divergence or convergencerelative to the axis of flow of the molten polymer through the die.

DESCRIPTION OF THE PREFERRED EMBODIMENT Dies

The molten ethylene polymer is extruded through a die, preferably anannular die, having a die gap greater than about 50 mils to less thanabout 200 mils. Extruding a molten ethylene polymer through a die havinga die gap of greater than about 50 mils to less than about 120 mils, isdescribed in U.S. patent application Ser. No. 892,324 filed Mar. 31,1978, and refiled as Ser. No. 012,795 on Feb. 16, 1979 in the names ofW. A. Fraser et al. and entitled A Process for Making Film From LowDensity Ethylene Hydrocarbon Copolymer.

The die which may be used in the present invention may be a spiralannulus die, rod die, etc.

FIG. 1 is a cross sectional view of a spiral/spider annulus die throughwhich the molten thermoplastic ethylene polymer is extruded. Die block,2, contains channels, 3. As the molten thermoplastic ethylene polymer isextruded it spreads out as it passes into the die channels. Dimension,b, is about 140 mils and dimension, a, is about 40 mils. The diameter ofthe die, d, is about 1 to 72 inches and preferably, from 6 to 32 inches.The die gap, c, is about 100 mils.

FIG. 2 is a cross section of a spiral die showing the spiral section, j,land entry section, h, die land, g, and die lip, e and f. Dimensions eand f are about 0.5 inches, g is about 2 inches, h is about 4 inches andj is about 6 inches.

FIG. 3 shows four different designs of die lips. 3i shows a die whereinthe die lip is divergent. Angle α is from about 1° to about 45°.Dimension k is from about 50 to about 200 mils; dimension, m, is fromabout 0.050 to 1.5 inches, while dimension, n, is from about 0.010 to0.110 inches. Die 3ii has one convergent die lip. Angle β is from about5° to about 50°. Die 3iii has both die lips convergent, and die 3iv hasboth die lips divergent.

In the practice of this invention, the angle of divergence orconvergence is in the area defined by the die land g, and/or die lip land f, as illustrated in FIG. 2. The polymer melt entering the die isdistributed around the die in the spiral distribution (or otherdistributing system such as is found in a spider die, for example) andland entry area to form an annular flow to the die land.

The use of a die as illustrated in FIG. 2 allows improvement in flowuniformity by using length, g, as a constriction; by combining a properconstriction and a die lip and/or die land geometry, uniform polymermelt, free of sharkskin melt fracture, and of good flow uniformity, canbe obtained.

When at least a portion of one surface of the die lip and/or die land isat a convergent angle, the area after the die l and may be divergent,preceeding the final converging section.

It is preferable to have an entry angle into the die land. This anglemay be about 5° to about 20°.

FILM EXTRUSION I. Blown Film Extrusion

The films herein may be extruded by tubular blown film extrusionprocess. In this process a narrow molecular weight distribution polymeris melt extruded through an extruder. This extruder may have anextrusion screw therein with a length to diameter ratio of between 15:1to 21:1, as described in U.S. patent application Ser. No. 940,005, filedSept. 6, 1978 in the names of John C. Miller et al and entitled "AProcess For Extruding Ethylene Polymers". This application describesthat this extrusion screw contains a feed, transition and meteringsection. Optionally, the extrusion screw can contain a mixing sectionsuch as that described in U.S. Pat. Nos. 3,486,192; 3,730,492 and3,756,574, which are incorporated herein by reference. Preferably, themixing section is placed at the screw tip.

The extruder which may be used herein is a 24:1. The extrusion screw ofthe present invention may be substituted directly for the 24/1 length todiameter extrusion screw. Alternatively, when, for example, an extrusionscrew of a length to diameter ratio of 18/1 is used in place of the 24/1extrusion screw, the remaining space in the extrusion barrel can bepartially filled with various types of plugs, torpedoes, or staticmixers to reduce residence time of the polymer melt. Also, the barrel ofthe extruder can be such so as to accomodate the 18/1 length to diameterextrusion screw directly.

The molten polymer is then extruded through a die, as will hereinafterbe described.

The polymer is extruded at a temperature of about 325° to about 500° F.The polymer is extruded in an upward vertical direction in the form of atube although it can be extruded downward or even sideways. Afterextrusion of the molten polymer through the annular die, the tubularfilm is expanded to the desired extent, cooled, or allowed to cool andflattened. The tubular film is flattened by passing the film through acollapsing frame and a set of nip rolls. These nip rolls are driven,thereby providing means for withdrawing the tubular film away from theannular die.

A positive pressure of gas, for example, air or nitrogen, is maintainedinside the tubular bubble. As is known in the operation of conventionalfilm processes, the pressure of the gas is controlled to give thedesired degree of expansion of the tubular film. The degree ofexpansion, as measured by the ratio of the fully expanded tubecircumference to the circumference of the die annulus, is in the range1/1 to 6/1 and preferably, 1/1 to 4/1. The tubular extrudate is cooledby conventional techniques such as, by air cooling, water quench ormandrel.

The drawdown characteristics of the polymers herein are excellent.Drawdown, defined as the ratio of the die gap to the product of filmgauge and blow up ratio, is kept greater than about 2 to less than about250 and preferably greater than about 25 to less than about 150. Verythin gauge films can be produced at high drawdown from these polymerseven when said polymer is highly contaminated with foreign particlesand/or gel. Thin gauge films greater than about 0.5 mils can beprocessed to exhibit ultimate elongations MD greater than about 400% toabout 700% and TD greater than about 500% to about 700%. Furthermore,these films are not perceived as "splitty". "Splittiness" is aqualitative term which describes the notched tear response of a film athigh deformation rates. "Splittiness" reflects crack propagation rate.It is an end-use characteristic of certain types of film and is not wellunderstood from a fundamentals perspective.

As the polymer exits the annular die, the extrudate cools and itstemperature falls below its melting point and it solidifies. The opticalproperties of the extrudate change as crystallization occurs and a frostline is formed. The position of this frost line above the annular die isa measure of the cooling rate of the film. This cooling rate has a verymarked effect on the optical properties of the film produced herein.

The ethylene polymer can also be extruded in the shape of a rod or othersolid cross section using the same die geometry for only the externalsurface. Additionally, the ethylene polymer can also be extruded intopipe through annular dies.

II. Slot Cast Film Extrusion

The films herein may also be extruded by slot cast film extrusion. Thisfilm extrusion method is well known in the art and comprises extruding asheet of molten polymer through a slot die and then quenching theextrudate using, for example, a chilled casting roll or water bath. Thedie will hereinafter be described. In the chill roll process, film maybe extruded horizontally and layed on top of the chill roll or it may beextruded downward and drawn under the chill roll. Extrudate coolingrates in the slot cast process are very high. Chill roll or water batchquenching is so fast that as the extrudate cools below its meltingpoint, crystallites nucleate very rapidly, supramolecular structureshave little time to grow and spherulites are held to a very small size.The optical properties of slot cast film are vastly improved over thosecharacterizing films using the slower cooling rate, tubular blown filmextrusion process. Compound temperatures in the slot cast film extrusionprocess generally run much higher than those typifying the tubular blownfilm process. Melt strength is not a process limitation in this filmextrusion method. Both shear viscosity and extensional viscosity arelowered. Film can generally be extruded at higher output rate thanpracticed in the blown film process. The higher temperatures reduceshear stresses in the die and raise the output threshold for meltfracture.

FILM

The film produced by the method of the present invention has a thicknessof greater than about 0.10 mils to about 20 mils, preferably greaterthan about 0.10 to 10 mils, most preferably greater than about 0.10 to4.0 mils. The 0.10 to 4.0 mil film is characterized by the followingproperties: a puncture resistance value of greater than about 7.0in-lbs/mil; an ultimate elongation of greater than about 400%; a thermalshrinkage of less than 3% after heating to 105°-110° C. and cooling toroom temperature; tensile impact strength of greater than about 500 toabout 2000 ft-lbs/in³ and tensile strength greater than about 2000 toabout 7000 psi.

Various conventional additives such as slip agents, antiblocking agents,and antioxidants can be incorporated in the film in accordance withconventional practice.

THE ETHYLENE POLYMERS

The copolymers which may be prepared in the process of the presentinvention are copolymers of a major mol percent (≧90%) of ethylene, anda minor mol percent (≦10%) of one or more C₃ to C₆ alpha olefins. The C₃to C₆ alpha olefins should not contain any branching on any of theircarbon atoms which is closer than the fourth carbon atom. The preferredC₃ to C₆ alpha olefins are propylene, butene-1, pentene-1 and hexene-1.

The ethylene polymers have a melt flow ratio of ≧18 to ≦32, andpreferably of ≧22 to ≦32. The melt flow ratio value is another means ofindicating the molecular weight distribution of a polymer. The melt flowratio (MFR) range of ≧22 to ≦32 thus corresponds to a Mw/Mn value rangeof about 2.7 to 4.1. The polymers herein include a Mw/Mn value in therange of about 2.2 to 4.1.

The homopolymers have a density of about ≧0.958 to ≦0.972 and preferablyof about ≧0.961 to ≦0.968.

The copolymers have a density of about ≧0.91 to ≦0.96 and preferably≧0.917 to ≦0.955, and most preferably, of about ≧0.917 to ≦0.935. Thedensity of the copolymer, at a given melt index level for the copolymer,is primarily regulated by the amount of the C₃ to C₆ comonomer which iscopolymerized with the ethylene. In the absence of the comonomer, theethylene would homopolymerize with the catalyst of the present inventionto provide homopolymers having a density of about ≧0.96. Thus, theaddition of progressively larger amounts of the comonomers to thecopolymers results in a progressive lowering of the density of thecopolymer. The amount of each of the various C₃ to C₆ comonomers neededto achieve the same result will vary from monomer to monomer, under thesame reaction conditions.

Thus, to achieve the same results, in the copolymers, in terms of agiven density, at a given melt index level, larger molar amounts of thedifferent comonomers would be needed in the order of C₃ >C₄ >C₅ >C₆.

The ethylene polymers of the present invention have an unsaturated groupcontent of ≦1, and usually ≧0.1 to ≦0.3, C=C/1000 carbon atoms, and acyclohexane extractables content of less than about 3, and preferablyless than about 2, weight percent.

The homopolymers of the present invention are granular materials whichhave an average particle size of the order of about 0.005 to about 0.06inches, and preferably of about 0.02 to about 0.04 inches, in diameter.The particle size is important for the purposes of readily fluidizingthe polymer particles in the fluid bed reactor, as described below. Thehomopolymers of the present invention have a settled bulk density ofabout 15 to 32 pounds per cubic foot.

The homopolymers and copolymers of the present invention are useful formaking film.

For film making purposes the preferred copolymers of the presentinvention are those having a density of about ≧0.917 to ≦0.924; amolecular weight distribution (Mw/Mn) of ≧2.7 to ≦3.6, and preferably ofabout ≧2.8 to 3.1; and a standard melt index of ≧0.5 to ≦5.0 andpreferably of about ≧1.0 to ≦4.0. The films have a thickness of >0 to≦10 mils and preferably of >0 to ≦5 mils.

HIGH ACTIVITY CATALYST

The compounds used to form the high activity catalyst used in thepresent invention comprise at least one titanium compound, at least onemagnesium compound, at least one electron donor compound, at least oneactivator compound and at least one inert carrier material, as definedbelow.

The titanium compound has the structure

    Ti(OR).sub.a X.sub.b

wherein R is a C₁ to C₁₄ aliphatic or aromatic hydrocarbon radical, orCOR' where R' is a C₁ to C₁₄ aliphatic or aromatic hydrocarbon radical,X is Cl, Br or I, a is 0 or 1, b is 2 to 4 inclusive and a+b=3 or 4.

The titanium compounds can be used individually or in combinationsthereof, and would include TiCl₃, TiCl₄, Ti(OCH₃)Cl₃, Ti(OC₆ H₅)Cl₃,Ti(OCOCH₃)Cl₃ and Ti(OCOC₆ H₅)Cl₃.

The magnesium compound has the structure

    MgX.sub.2

wherein X is Cl, Br or I. Such magnesium compounds can be usedindividually or in combinations thereof and would include MgCl₂, MgBr₂and MgI₂. Anhydrous MgCl₂ is the particularly preferred magnesiumcompound.

About 0.5 to 56, and preferably about 1 to 10, mols of the magnesiumcompound are used per mol of the titanium compound in preparing thecatalysts employed in the present invention.

The titanium compound and the magnesium compound should be used in aform which will facilitate their dissolution in the electron donorcompound, as described herein below.

The electron donor compound is an organic compound which is liquid at25° C. and in which the titanium compound and the magnesium compound arepartially or completely soluble. The electron donor compounds are known,as such, or as Lewis bases.

The electron donor compounds would include such compounds as alkylesters of aliphatic and aromatic carboxylic acids, aliphatic ethers,cyclic ethers and aliphatic ketones. Among these electron donorcompounds the preferable ones are alkyl esters of C₁ to C₄ saturatedaliphatic carboxylic acids; alkyl esters of C₇ to C₈ aromatic carboxylicacids; C₂ to C₈, and preferably C₃ to C₄, aliphatic ethers; C₃ to C₄cyclic ethers, and preferably C₄ cyclic mono- or di-ether; C₃ to C₆, andpreferably C₃ to C₄, aliphatic ketones; The most preferred of theseelectron donor compounds would include methyl formate, ethyl acetate,butyl acetate, ethyl ether, hexyl ether, tetrahydrofuran, dioxane,acetone and methyl isobutyl ketone.

The electron donor compounds can be used individually or in combinationsthereof.

About 2 to 85, and preferably about 3 to 10 mols of the electron donorcompound are used per mol of Ti.

The activator compound has the structure

    Al(R").sub.c X'.sub.d H.sub.e

wherein X' is Cl or OR"', R" and R"' are the same or different and areC₁ to C₁₄ saturated hydrocarbon radicals, d is 0 to 1.5, e is 1 or 0 andc+d+e=3.

Such activator compounds can be used individually or in combinationsthereof and would include Al(C₂ H₅)₃, Al(C₂ H₅)₂ Cl, Al(i-C₄ H₉)₃, Al₂(C₂ H₅)₃ Cl₃, Al(i-C₄ H₉)₂ H, Al(C₆ H₁₃)₃, Al(C₂ H₅)₂ H and Al(C₂ H₅)₂(OC₂ H₅).

About 10 to 400, and preferably about 10 to 100, mols of the activatorcompound are used per mol of the titanium compound in activating thecatalyst employed in the present invention.

The carrier materials are solid, particulate materials which are inertto the other components of the catalyst composition, and to the otheractive components of the reaction system. These carrier materials wouldinclude inorganic materials such as oxides of silicon and aluminum andmolecular sieves, and organic materials such as olefin polymers such aspolyethylene. The carrier materials are used in the form of dry powdershaving an average particle size of about 10 to 250, and preferably ofabout 50 to 150 microns. These materials are also preferably porous andhave a surface area of ≧3, and preferably of ≧50, square meters pergram. The carrier material should be dry, that is, free of absorbedwater. This is normally done by heating or pre-drying the carriermaterial with a dry inert gas prior to use. The inorganic carrier mayalso be treated with about 1 to 8 percent by weight of one or more ofthe aluminum alkyl compounds described above to further activate thecarrier.

CATALYST PREPARATION

The catalyst used in the present invention is prepared by firstpreparing a precursor composition from the titanium compound, themagnesium compound, and the electron donor compound, as described below.The carrier material can then be impregnated with precursor compositionand then treated with the activator compound in one or more steps asdescribed below. Alternatively the precursor composition can be treatedwith the carrier material and the activator compound in one or moresteps as described below.

The precursor composition is formed by dissolving the titanium compoundand the magnesium compound in the electron donor compound at atemperature of about 20° C. up to the boiling point of the electrondonor compound. The titanium compound can be added to the electron donorcompound before or after the addition of the magnesium compound, orconcurrent therewith. The dissolution of the titanium compound and themagnesium compound can be facilitated by stirring, and in some instancesby refluxing, these two compounds in the electron donor compound. Afterthe titanium compound and the magnesium compound are dissolved, theprecursor composition may be isolated by crystallization or byprecipitation with a C₅ to C₈ aliphatic or aromatic hydrocarbon such ashexane, isopentane or benzene.

The crystallized or precipitated precursor composition may be isolated,in the form of fine, free flowing particles having an average particlesize of about 10 to 100 microns and a settled bulk density of about 18to 33 pounds per cubic foot.

When thus made as disclosed above the precursor composition has theformula

    Mg.sub.m Ti.sub.1 (OR).sub.n X.sub.p [ED].sub.q

wherein

ED is the electron donor compound,

m is ≧0.5 to ≦56, and preferably ≧1.5 to ≦5,

n is 0.1 or 2

p is ≧2 to ≦116, and preferably ≧6 to ≦14,

q is ≧2 to ≦85, and preferably ≧4 to ≦11,

R is a C₁ to C₁₄ aliphatic or aromatic hydrocarbon radical, or COR'wherein R' is a C₁ to C₁₄ aliphatic or aromatic hydrocarbon radical and,

X is Cl, Br or I.

The precursor composition may then be impregnated, in a weight ratio ofabout 0.033 to 1, and preferably about 0.1 to 0.33, parts of theprecursor composition into one part by weight of the carrier material.

The impregnation of the dried (activated) support with the precursorcomposition may be accomplished by dissolving the precursor compositionin the electron donor compound, and by then admixing the support withthe dissolved precursor composition so as to allow the precursorcomposition to impregnate the support. The solvent is then removed bydrying at temperatures of ≦70° C.

The support may also be impregnated with the precursor composition byadding the support to a solution of the chemical raw materials used toform the precursor composition in the electron donor compound, withoutisolating the precursor composition from such solution. The excesselectron donor compound is then removed by drying at temperatures of≦70° C.

Alternatively, the precursor composition can be diluted with the carriermaterial. The dilution of the precursor composition can be accomplishedbefore the precursor composition is partially or completely activated,as disclosed below, or concurrent with such activation. The dilution ofthe precursor composition is accomplished by mechanically mixing orblending about 0.033 to 1, and preferably about 0.1 to 0.33, parts ofthe precursor composition with one part by weight of the carriermaterial.

ACTIVATION OF PRECURSOR COMPOSITION

In order to be used in the process of the present invention theprecursor composition must be fully or completely activated, that is, itmust be treated with sufficient activator compound to transform the Tiatoms in the precursor composition to an active state.

It has been found that, in order to prepare a useful catalyst it isnecessary to conduct the activation in such a way that, at least, thefinal activation stage must be conducted in the absence of solvent so asto avoid the need for drying the fully active catalyst to remove solventtherefrom. The activation procedure is hereafter described as to theimpregnated precursor composition (A) and wherein the precursorcomposition is diluted with the carrier material (B).

A. Activation of Impregnated Precursor Composition

The activation is conducted in at least two stages. In the first stagethe precursor composition, impregnated in the silica, is reacted with,and partially reduced by, enough activator compound so as to provide apartially activated precursor composition which has an activatorcompound/Ti molar ratio of about >0 to <10:1 and preferably of about 4to about 8:1. This partial reduction reaction is preferably carried outin a hydrocarbon solvent slurry followed by drying of the resultingmixture, to remove the solvent, at temperatures between 20 to 80, andpreferably of 50° to 70° C. In this partial activation procedure theactivator compound may be used while absorbed on the carrier materialused as the support for the precursor composition. The resulting productis a free-flowing solid particulate material which can be readily fed tothe polymerization reactor. The partially activated and impregnatedprecursor composition, however, is at best, weakly active, as apolymerization catalyst in the process of the present invention. Inorder to render the partially activated and impregnated precursorcomposition active for ethylene polymerization purposes, additionalactivator compound must also be added to the polymerization reactor tocomplete, in the reactor, the activation of the precursor composition.The additional activator compound and the partially activatedimpregnated precursor composition are preferably fed to the reactorthrough separate feed lines. The additional activator compound may besprayed into the reactor in the form of a solution thereof in ahydrocarbon solvent such as isopentane, hexane, or mineral oil. Thissolution usually contains about 2 to 30 weight percent of the activatorcompound. The additional activator compound is added to the reactor insuch amounts as to provide, in the reactor, with the amounts ofactivator compound and titanium compound fed with the partiallyactivated and impregnated precursor compositiion, a total Al/Ti molarratio of ≧10 to 400 and preferable of about 15 to 60. The additionalamounts of activator compound added to the reactor, react with, andcomplete the activation of, the titanium compound in the reactor.

B. Activation where Precursor is Diluted with Carrier Material

Two procedures have been developed to accomplish this result. In oneprocedure, the precursor composition is completely activated, outsidethe reactor, in the absence of solvent, by dry blending the precursorcomposition with the activator compound. In this dry blending procedurethe activator compound is preferably used while absorbed on a carriermaterial. This procedure has a disadvantage, however, in that theresulting dry, fully activated catalyst is pyrophoric where itcontains >10 weight percent of the activator compound.

In the second, and preferred, of such catalyst activation procedures,the precursor composition is partially activated outside thepolymerization reactor with activator compound in a hydrocarbon slurry,the hydrocarbon solvent is removed by drying and the partially activatedprecursor composition is fed to the polymerization reactor where theactivation is completed with additional activator compound.

Thus, in the dry blending catalyst making procedure the solidparticulate precursor composition is added to and evenly blended withsolid particles of porous carrier material wherein the activatorcompound is absorbed. The activator compound is absorbed on the carriermaterial, from a hydrocarbon solvent solution of the activator compound,so as to provide a loading of about 10 to 50 weight percent of activatorcompound on 90 to 50 weight percent of carrier material. The amounts ofthe precursor composition, activator compound and carrier material thatare employed are such as to provide the desired Al/Ti molar ratios andto provide a final composition having a weight ratio of precursorcomposition to carrier material of less than about 0.50, and preferablyof less than about 0.33. This amount of carrier material thus providesthe necessary dilution therewith of the activated catalyst so as toprovide the desired control of the polymerization activity of thecatalyst in the reactor. Where the final compositions contain about ≧10weight percent of the activator compound, they will be pyrophoric,During the dry blending operation, which may be conducted at ambient(25° C.) or lower temperatures, the dry mixture is well agitated toavoid any heat build-up during the ensuing reduction reaction which isexothermic, initially. The resulting catalyst is thus completely reducedand activated and can be fed to, and used as such in, the polymerizationreactor. It is a free-flowing particulate material.

In the second, and preferred catalyst activation procedure, theactivation is conducted in at least two stages. In the first stage thesolid particulate precursor composition, diluted with carrier material,is reacted with and partially reduced by enough activator compound so asto provide a partially activated precursor composition which has anactivator compound/Ti molar ratio of about 1 to 10:1 and preferably ofabout 4 to 8:1. This partial reduction reaction is preferably carriedout in a hydrocarbon solvent slurry followed by drying of the resultingmixture to remove the solvent, at temperatures between 20 to 80, andpreferably of 50° to 70° C. In this partial activation procedure theactivator compound may be used while absorbed on the carrier materialused to dilute the activator compound. The resulting product is afree-flowing solid particulate material which can be readily fed to thepolymerization reactor. The partially activated precursor composition,however, is, at best, weakly active as a polymerization catalyst in theprocess of the present invention. In order to render the partiallyactivated precursor composition active for ethylene polymerizationpurposes, additional activator compound must also be added to thepolymerization reactor to complete, in the reactor, the activation ofthe percursor composition. The additional activator compound and thepartially activated precursor composition are preferably fed to thereactor through separate feed lines. The additional activator compoundmay be sprayed into the reactor in the form of a solution thereof in ahydrocarbon solvent such as isopentane, hexane, or mineral oil. Thissolution usually contains about 2 to 30 weight percent of the activatorcompound. The activator compound may also be added to the reactor insolid form, by being absorbed on a carrier material. The carriermaterial usually contains 10 to 50 weight percent of the activator forthis purpose. The additional activator compound is added to the reactorin such amounts as to provide, in the reactor, with the amounts ofactivator compound and titanium compound fed with the partiallyactivated precursor composition, a total A1/Ti molar ratio of about 10to 400 and preferably of about 15 to 60. The additional amounts ofactivator compound added to the reactor, react with, and complete theactivation of, the titanium compound in the reactor.

In a continuous gas phase process, such as the fluid bed processdisclosed below, discrete portions of the partially or completelyactivated precursor composition or discrete portions of the partiallyactivated precursor composition impregnated on the support arecontinuously fed to the reactor, with discrete portions of anyadditional activation of the partially activated precursor composition,during the continuing polymerization process in order to replace activecatalyst sites that are expended during the course of the reaction.

The Polymerization Reaction

The polymerization reaction is conducted by contacting a stream of themonomer(s), in a gas phase process, such as in the fluid bed processdescribed below, and substantially in the absence of catalyst poisonssuch as moisture, oxygen, CO, CO₂, and acetylene with a catalyticallyeffective amount of the completely activated precursor composition (thecatalyst) which may be impregnated on a support at a temperature and ata pressure sufficient to initiate the polymerization reaction.

In order to achieve the desired density ranges in the copolymers it isnecessary to copolymerize enough of the ≧C₃ comonomers with ethylene toachieve a level of ≧0 to 10 mol percent of the C₃ to C₆ comonomer in thecopolymer. The amount of comonomer needed to achieve this result willdepend on the particular comonomer(s) employed.

There is provided below a listing of the amounts, in mols, of variouscomonomers that must be copolymerized with ethylene in order to providepolymers having the desired density range at any given melt index. Thelisting also indicates the relative molar concentration, of suchcomonomer to ethylene, which must be present in the gas stream ofmonomers which is fed to the reactor.

    ______________________________________                                                                 Gas Stream                                                       mol % needed Comonomer/Ethylene                                   Comonomer   in copolymer molar ratio                                          ______________________________________                                        propylene   >0 to 10     >0 to 0.9                                            butene-1    >0 to 7.0    >0 to 0.7                                            pentene-1   >0 to 6.0     >0 to 0.45                                          hexene-1    >0 to 5.0    >0 to 0.4                                            ______________________________________                                    

A fluidized bed reaction system which can be used in the practice of theprocess of the present invention is illustrated in FIG. 4. Withreference thereto the reactor 10 consists of a reaction zone 12 and avelocity reduction zone 14.

The reaction zone 12 comprises a bed of growing polymer particles,formed polymer particles and a minor amount of catalyst particlesfluidized by the continuous flow of polymerizable and modifying gaseouscomponents in the form of make-up feed and recycle gas through thereaction zone. To maintain a viable fluidized bed, the mass gas flowrate through the bed must be above the minimum flow required forfluidization, and preferably from about 1.5 to about 10 times G_(mf) andmore preferably from about 3 to about 6 times G_(mf). G_(mf) is used inthe accepted form as the abbreviation for the minimum mass gas flowrequired to achieve fluidization, C. Y. Wen and Y. H. Yu, "Mechanics ofFluidization", Chemical Engineering Progress Symposium Series, Vol. 62,p. 100-111 (1966).

It is essential that the bed always contains particles to prevent theformation of localized "hot spots" and to entrap and distribute theparticulate catalyst throughout the reaction zone. On start up, thereaction zone is usually charged with a base of particulate polymerparticles before gas flow is initiated. Such particles may be identicalin nature to the polymer to be formed or different therefrom. Whendifferent, they are withdrawn with the desired formed polymer particlesas the first product. Eventually, a fluidized bed of the desired polymerparticles supplants the start-up bed.

The partially or completely activated precursor compound (the catalyst)used in the fluidized bed is preferably stored for service in areservoir 32 under a blanket of a gas which is inert to the storedmaterial, such as nitrogen and argon.

Fluidization is achieved by a high rate of gas recycle to and throughthe bed, typically in the order of about 50 times the rate of feed ofmake-up gas. The fluidized bed has the general appearance of a densemass of viable particles in possible free-vortex flow as created by thepercolation of gas through the bed. The pressure drop through the bed isequal to or slightly greater than the mass of the bed divided by thecross-sectional area. It is thus dependent on the geometry of thereactor.

Make-up gas is fed to the bed at a rate equal to the rate at whichparticulate polymer product is withdrawn. The composition of the make-upgas is determined by a gas analyzer 16 positioned above the bed. The gasanalyzer determines the composition of the gas being recycled and thecomposition of the make-up gas is adjusted accordingly to maintain anessentially steady state gaseous composition within the reaction zone.

To insure complete fluidization, the recycle gas and, where desired,part of the make-up gas are returned to the reactor at point 18 belowthe bed. There exists a gas distribution plate 20 above the point ofreturn to aid fluidizing the bed.

The portion of the gas stream which does not react in the bedconstitutes the recycle gas which is removed from the polymerizationzone, preferably by passing it into a velocity reduction zone 14 abovethe bed where entrained particles are given an opportunity to drop backinto the bed. Particle return may be aided by a cyclone 22 which may bepart of the velocity reduction zone or exterior thereto. Where desired,the recycle gas may then be passed through a filter 24 designed toremove small particles at high gas flow rates to prevent dust fromcontacting heat transfer surfaces and compressor blades.

The recycle gas is then compressed in a compressor 25 and then passedthrough a heat exchanger 26 wherein it is stripped of heat of reactionbefore it is returned to the bed. By constantly removing heat ofreaction, no noticeable temperature gradient appears to exist within theupper portion of the bed. A temperature gradient will exist in thebottom of the bed in a layer of about 6 to 12 inches, between thetemperature of the inlet gas and the temperature of the remainder of thebed. Thus, it has been observed that the bed acts to almost immediatelyadjust the temperature of the recycle gas above this bottom layer of thebed zone to make it conform to the temperature of the remainder of thebed thereby maintaining itself at an essentially constant temperatureunder steady state conditions. The recycle is then returned to thereactor at its base 18 and to the fluidized bed through distributionplate 20. The compressor 25 can also be placed upstream of the heatexchanger 26.

The distribution plate 20 plays an important role in the operation ofthe reactor. The fluidized bed contains growing and formed particulatepolymer particles as well as catalyst particles. As the polymerparticles are hot and possible active, they must be prevented fromsettling, for if a quiescent mass is allowed to exist, any activecatalyst contained therein may continue to react and cause fusion.Diffusing recycle gas through the bed at a rate sufficient to maintainfluidization at the base of the bed is, therefore, important. Thedistribution plate 20 serves this purpose and may be a screen, slottedplate, perforated plate, a plate of the bubble cap type, and the like.The elements of the plate may all be stationary, or the plate may be ofthe mobile type disclosed in U.S. Pat. No. 3,298,792. Whatever itsdesign, it must diffuse the recycle gas through the particles at thebase of the bed to keep them in a fluidized condition, and also serve tosupport a quiescent bed of resin particles when the reactor is not inoperation. The mobile elements to the plate may be used to dislodge anypolymer particles entrapped in or on the plate.

Hydrogen may be used as a chain transfer agent in the polymerizationreaction of the present invention. The ratio of hydrogen/ethyleneemployed will vary between about 0 to about 2.0 moles of hydrogen permole of the monomer in the gas stream.

Any gas inert to the catalyst and reactants can also be present in thegas stream. The activator compound is preferably added to the reactionsystem at the hottest portion of the gas which is usually downstreamfrom heat exchanger 26. Thus, the activator may be fed into the gasrecycle system from dispenser 27 thru line 27A.

Compounds of the structure Zn(R_(a))(R_(b)), wherein R_(a) and R_(b) arethe same or different C₁ to C₁₄ aliphatic or aromatic hydrocarbonradicals, may be used in conjunction with hydrogen, with the catalystsof the present invention as molecular weight control or chain transferagents, that is, to increase the melt index values of the copolymersthat are produced. About 0 to 50, and preferably about 20 to 30, mols ofthe Zn compound (as Zn) would be used in the gas stream in the reactorper mol of titanium compound (as Ti) in the reactor. The zinc compoundwould be introduced into the reactor preferably in the form of a dilutesolution (2 to 30 weight percent) in hydrocarbon solvent or absorbed ona solid diluent material, such as silica, of the types described above,in amounts of about 10 to 50 weight percent. These compositions tend tobe pyrophoric. The zinc compound may be added alone, or with anyadditional portions of the activator compound that are to be added tothe reactor from a feeder, not shown, which could be positioned adjacentdispenser 27, near the hottest portion of the gas recycle system.

It is essential to operate the fluid bed reactor at a temperature belowthe sintering temperature of the polymer particles. To insure thatsintering will not occur, operating temperatures below the sinteringtemperature are desired. For the production of ethylene copolymers inthe process of the present invention an operating temperature of about30 to 115° C. is preferred, and a temperature of about 75 to 95° C. ismost preferred. Temperatures of about 75 to 95° C. are used to prepareproducts having a density of about 0.91 to 0.92, and temperatures ofabout 80 to 100° C. are used to prepare products having a density ofabout >0.92 to 0.94, and temperatures of about 90 to 115° C. are used toprepare products having a density of about >0.94 to 0.96.

The fluid bed reactor is operated at pressures of up to about 1000 psi,and is preferably operated at a pressure of from about 150 to 350 psi,with operation at the higher pressures in such ranges favoring heattransfer since an increase in pressure increases the unit volume heatcapacity of the gas.

The partially or completely activated precursor composition is injectedinto the bed at a rate equal to its consumption at a point 30 which isabove the distribution plate 20. Preferably, the catalyst is injected ata point located about 1/4 to 3/4 up the side of the bed. Injecting thecatalyst at a point above the distribution plate is an important featureof this invention. Since the catalysts used in the practice of theinvention are highly active, injection of the fully activated catalystinto the area below the distribution plate may cause polymerization tobegin there and eventually cause plugging of the distribution plate.Injection into the viable bed, instead, aids in distributing thecatalyst throughout the bed and tends to preclude the formation oflocalized spots of high catalyst concentration which may result in theformation of "hot spots".

A gas which is inert to the catalyst such as nitrogen or argon is usedto carry the partially or completely reduced precursor composition, andany additional activator compound or non-gaseous chain transfer agentthat is needed, into the bed.

The production rate of the bed is controlled by the rate of catalystinjection. The production rate may be increased by simply increasing therate of catalyst injection and decreased by reducing the rate ofcatalyst injection.

Since any change in the rate of catalyst injection will change the rateof generation of the heat of reaction, the temperature of the recyclegas is adjusted upwards or downwards to accomodate the change in rate ofheat generation. This insures the maintenance of an essentially constanttemperature in the bed. Complete instrumentation of both the fluidizedbed and the recycle gas cooling system, is, of course, necessary todetect any temperature change in the bed so as to enable the operator tomake a suitable adjustment in the temperature of the recycle gas.

Under a given set of operating conditions, the fluidized bed ismaintained at essentially a constant height by withdrawing a portion ofthe bed as product at a rate equal to the rate of formation of theparticulate polymer product. Since the rate of heat generation isdirectly related to product formation, a measurement of the temperaturerise of the gas across the reactor (the difference between inlet gastemperature and exit gas temperature) is determinative of the rate ofparticulate polymer formation at a constant gas velocity.

The particulate polymer product is preferably continuously withdrawn ata point 34 at or close to the distribution plate 20 and in suspensionwith a portion of the gas stream which is vented before the particlessettle to preclude further polymerization and sintering when theparticles reach their ultimate collection zone. The suspending gas mayalso be used, as mentioned above, to drive the product of one reactor toanother reactor.

The particulate polymer product is conveniently and preferably withdrawnthrough the sequential operation of a pair of timed valves 36 and 38defining a segregation zone 40. While valve 38 is closed, valve 36 isopened to emit a plug of gas and product to the zone 40 between it andvalve 36 which is then closed. Valve 38 is then opened to deliver theproduct to an external recovery zone. Valve 38 is then closed to awaitthe next product recovery operation.

Finally, the fluidized bed reactor is equipped with an adequate ventingsystem to allow venting the bed during start up and shut down. Thereactor does not require the use of stirring means and/or wall scrapingmeans.

The catalyst system of this invention appears to yield a fluid bedproduct having an average particle size between about 0.005 to about0.06 inches and preferably about 0.02 to about 0.04 inches.

The properties of the polymers produced in the Examples were determinedby the following test methods:

Density

For materials having a density <0.940, ASTM-1505 procedure is used andplaque is conditioned for one hour at 100° C. to approach equilibriumcrystallinity.

For materials having a density of ≧0.940, a modified procedure is usedwherein the test plaque is conditioned for one hour at 120° C. toapproach equilibrium crystallinity and is then quickly cooled to roomtemperature. All density values are reported as grams/cm³. All densitymeasurements are made in a density gradient column.

Melt Index (MI)

ASTM D-1238--Condition E--Measured at 190° C.--reported as grams per 10minutes.

Flow Rate (HLMI)

ASTM D-1238--Condition F--Measured at 10 times the weight used in themelt index test above. ##EQU1##

productivity

a sample of the resin product is ashed, and the weight % of ash isdetermined; since the ash is essentially composed of the catalyst, theproductivity is thus the pounds of polmer produced per pound of totalcatalyst consumed. The amount of Ti, Mg and Cl in the ash are determinedby elemental analysis.

n-hexane extractables

(FDA test used for polyethylene film intended for food contactapplications).

A 200 square inch sample of 1.5 mil gauge film is cut into stripsmeasuring 1"×6" and weighted to the nearest 0.1 mg. The strips areplaced in a vessel and extracted with 300 ml of n-hexane at 50°±1° C.for 2 hours. The extract is then decanted into tared culture dishes.After drying the extract in a vacuum desiccator, the culture dish isweighed to the nearest 0.1 mg. The extractables, normalized with respectto the original sample weight, is then reported as the weight fractionof n-hexane extractables.

bulk density

The resin is poured via 3/8" diameter funnel into a 100 ml graduatedcylinder to 100 ml line without shaking the cylinder, and weighed bydifference.

Molecular Weight Distribution (Mw/Mn)

Gel Permeation Chromatography For resins with density <0.94: StyrogelPacking: (Pore Size Sequence is 10⁷, 10⁵, 10⁴, 10³, 60 A°) Solvent isPerchloroethylene at 117° C. For resins with density 24 0.94: StyrogelPacking: (Pore Size Sequence is 10⁷, 10⁶, 10⁵, 10⁴, 60 A°) Solvent isortho dichloro benzene at 135° C.

Detection for all resins; Infra red at 3.45

The following Examples are designed to illustrate the process of thepresent invention and are not intended as a limitation upon the scopethereof.

I. PREPARATION OF IMPREGNATED PRECURSOR

In a 12 1 flask equipped with a mechanical stirrer are placed 41.8 g(0.439 mol) anhydrous MgCl₂ and 2.5 1 tetrahydrofuran (THF). To thismixture, 27.7 g (0.184 mol) TiCl₄ is added dropwise over 1/2 hour. Itmay be necessary to heat the mixture to 60° C. for about 1/2 hour inorder to completely dissolve the material.

500 g of porous silica is added and the mixture stirred for 1/4 hour.The mixture is dried with a N₂ purge at 60° C. for about 3-5 hours toprovide a dry free flowing powder having the particle size of thesilica. The absorbed precursor composition has the formula

    TiMg.sub.3.0 Cl.sub.10 (THF).sub.6.7

Ib. Preparation of Impregnated Precursor from Preformed PrecursorComposition

In a 12 liter flask equipped with a mechanical stirrer, 146 g ofprecursor composition is dissolved in 2.5 liters dry THF. The solutionmay be heated to 60° C. in order to facilitate dissolution. 500 g ofporous silica is added and the mixture is stirred for 1/4 hour. Themixture is dried with a N₂ purge at ≦60° C. for about 3-5 hours toprovide a dry free flowing powder having the particle size of thesilica.

The precursor composition employed in this Procedure Ib. is formed as inProcdure Ia. except that it is recovered from the solution thereof inTHF by crystallization or precipitation.

The precursor composition may be analyzed at this point for Mg and Ticontent since some of the Mg and/or Ti compound may have been lostduring the isolation of the precursor composition. The empiricalformulas used herein in reporting the precursor compositions are derivedby assuming that the Mg and Ti still exist in the form of the compoundsin which they were first added to the electron donor compound. Theamount of electron donor is determined by chromatography.

II. ACTIVATION PROCEDURE

The desired weights of impregnated, precursor composition and activatorcompound are added to a mixing tank with sufficient amounts of anhydrousaliphatic hydrocarbon diluent such as isopentane to provide a slurrysystem.

The activator compound and precursor compound are used in such amountsas to provide a partially activated precursor composition which as anAl/Ti ratio of <0 to ≦10:1 and preferably of 4 to 8:1.

The contents of the slurry system are then thoroughly mixed at roomtemperature and at a atmospheric pressure for about 1/4 to 1/2 hour. Theresulting slurry is then dried under a purge of dry inert gas such asnitrogen or argon, at a atmospheric pressure and at a temperature of65°±10° C. to remove the hydrocarbon diuluent. This process usuallyrequires about 3 to 5 hours. The resulting catalyst is in the form of apartially activated precursor composition which is impregnated withinthe pores of the silica. The material is a free flowing particulatematerial having the size and shape of the silica. It is not pyrophoricunless the aluminum alkyl content exceeds a loading of 10 weightpercent. It is stored under a dry inert gas such as nitrogen or argonprior to future use. It is now ready for use by being injected into, andfully activated within, the polymerization reactor.

When additional activator compound is fed to the polymerization reactorfor the purpose of completing the activation of the precursorcomposition, it is fed into the reactor as a dilute solution in ahydrocarbon solvent such as isopentane. These dilute solutions containabout 5 to 30% by volume of the activator compound.

The activator compound is added to the polymerization reactor so as tomaintain the Al/Ti ratio in the reactor at a level of about ≧10 to 400:1and preferably of 15 to 60:1.

EXAMPLE 1 Preparation of Copolymer

Ethylene was copolymerized with propylene or butene-1 (propylene in Runs1 and 2 and butene-1 in Runs 3 to 14) in each of this series withcatalyst formed as described above and as activated by ActivationProcedure A to produce polymers having a density of ≦0.940. In eachcase, the partially activated precursor composition had an Al/Ti molratio of 4.4 to 5.8. The completion of the activation of the precursorcomposition in the polymerization reactor was accomplished with triethylaluminum (as a 5 weight % solution in isopentane in Runs 1 to 3 and 4 to14, and adsorbed in silica, 50/50 weight %, in Runs 4 and 5 so as toprovide the completely activated catalyst in the reactor with an Al/Timol ratio of about 29 to 140.

Each of the polymerization reaction was continuously conducted for >1hour after equilibrium was reached and under a pressure of about 300psia and a gas velocity of about 5 to 6 times G_(mf) in a fluid bedreactor system at a space time yield of about 3 to 6 lbs/hr/ft³ of bedspace. The reaction system was as described in the drawing above. It hasa lower section 10 feet high and 131/2 inches in (inner) diameter, andan upper section which was 16 feet high and 231/2 inches in (inner)diameter.

In several of the Runs zinc diethyl was added during the reaction (as a2.6 weight % solution in isopentane) to maintain a constant Zn/Ti molratio where the zinc diethyl was used, the triethyl aluminum was alsoadded as a 2.6 weight percent in isopentane.

Table A below lists, with respect to Runs 1 to 14 various operatingconditions employed in such examples i.e., the weight percent ofprecursor composition in the blend of silica and precursor composition;Al/Ti ratio in the partially activated precursor composition; Al/Tiratio maintained in the reactor; polymerization temperature; percent byvolume of ethylene in reactor; H₂ /ethylene mol ratio; comonomer(C_(x))/C₂ mol ratio in reactor; catalyst productivity and Zn/Ti molratio. Table B below lists properties of the granular virgin resins madein runs 1 to 14, i.e., density, melt index (M.I.); melt flow ratio(MFR); weight percent ash; Ti content (ppm), bulk density and averageparticle size.

                  TABLE A                                                         ______________________________________                                        Reaction Conditions For Runs 1 to 14                                                       Al/ti                                                                         ratio                                                                 Weight  in part.                                                                              Al/Ti                                                         %       act     ratio             H.sub.2 /C.sub.2                                                                    C/C.sub.2                        Run  pre-    pre-    in    Temp  Vol % mol   mol                              No   cursor  cursor  reactor                                                                             C.    C.sub.2                                                                             ratio ratio                            ______________________________________                                        1    8.3     5.8     40.5  90    41.7  0.492 0.486                            2    8.3     5.8     50.8  90    39.7  0.566 0.534                            3    20.1    4.50    88.3  85    56.3  0.148 0.450                            4    19.8    4.40    26.7  85    50.2  0.350 0.350                            5    19.8    4.40    26.7  80    54.1  0.157 0.407                            6    6.9     5.08    42.0  85    49.2  0.209 0.480                            7    6.9     5.08    33.6  85    46.5  0.208 0.482                            8    6.9     5.08    28.8  85    42.1  0.206 0.515                            10   8.3     5.8     124.6 90    45.1  0.456 0.390                            11   8.3     5.8     80.8  90    42.7  0.365 0.396                            12   8.3     5.8     52.0  90    48.4  0.350 0.397                            13   8.3     5.8     140.1 90    42.6  0.518 0.393                            14   8.3     5.8     63.5  90    40.8  0.556 0.391                            ______________________________________                                    

                  TABLE B                                                         ______________________________________                                        Properties of Polymers Made in Runs 1 to 14                                                                          average                                                                bulk   particle                               Run No Density   M.I.    MFR    density                                                                              size, inches                           ______________________________________                                        1      0.927     22.0    24.4   16.8   0.0230                                 2      0.929     24.0    23.4   17.5   0.0230                                 3      0.925     0.61    27.1   16.8   0.0300                                 4      0.931     12.0    26.7   16.8   0.0275                                 5      0.923     1.47    28.2   15.6   0.0404                                 6      0.919     3.41    25.9   16.8   0.0550                                 7      0.925     2.90    24.5   17.5   0.0590                                 8      0.919     3.10    24.6   16.2   0.0570                                 10     0.929     16.0    24.1   17.3   0.0230                                 11     9.929     15.3    24.0   16.6   0.0234                                 12     0.928     11.5    24.1   16.7   0.0248                                 13     0.929     20.7    24.3   17.3   0.0258                                 14     0.929     29.2    26.1   16.8   0.0206                                 ______________________________________                                    

EXAMPLE 2

An ethylene-butene copolymer prepared as in Example 1 and having adensity of 0.924 and a melt index of 2.0 was formed into a film of 1.5mil gauge by blown film extrusion using a 21/2 inch diameter 18:1 L/Dextrusion screw in a 24/1 extruder. The extrusion screw had a feedsection of 12.5 inches, transition section of 7.5 inches, a meteringsection of 20 inches, and a mixing section of 5 inches. The mixingsection was a fluted mixing section with the following characteristics:a diameter of 2.5 inches; 3.0 inch channels; channel radius of 0.541inches; mixing barrier land width of 0.25 inches; cleaning barrier landwidth of 0.20 inches; and a mixing barrier length of 4.5 inches. Thevoid in the barrel was filled by a plug 2.496 inches in diameter, 11.0inches long which contained a static mixer 9.0 inches long and 1.0 inchin diameter. Also, a 20/60/20 mesh screen pack and a three inch diameterdie were used. The die had a gap of 40 mils. The sides of the die wereparallel with the flow axis of the polymer melt. The melt temperature ofthe copolymer was about 400° F. Nip roll height was approximately 15 ft.Cooling was accomplished with a Venturi type air ring. All films wereprepared at a 2:1 blow-up ratio (ratio of bubble circumference to diecircumference). The rate of production of the film was 7.27lbs/hour/inch of die. Sharkskin melt fracture was measured using a 40×magnification microscope. In this procedure, the extrudate is lightedfrom the side. The microscope shows the transition from a low-shearglossy surface of the extrudate to a critical-shear, matted surface (theonset of sharkskin melt fracture) to high-shear, deep-ridge, sharkskinmelt fracture. A high level of sharkskin melt fracture was observedduring production of the film.

EXAMPLE 3

The procedure of Example 2 was exactly repeated except that melttemperature was about 380° F. and the rate of production of the film was4.14 pounds/hour/inch of die. A high level of sharkskin melt fracturewas observed during production of the film.

EXAMPLE 4

The procedure of Example 2 was exactly repeated except that the die hada gap of 80 mils, melt temperature was about 390° F. and the rate ofproduction of the film was 7.38 pounds/hour/inch of die. A low level ofsharkskin melt fracture was observed during production of the film.

EXAMPLE 5

The procedure of Example 2 was exactly repeated except that a die hadthe configuration as in FIG. 3i was used with angle α=4.57°, anddimensions k=80 mils, m=50 mils, and n=40 mils in FIG. 3i. Melttemperature was about 398° F. and the rate of production of the film was7.22 pounds/hour/inch of die. No melt fracture was observed duringproduction of the film.

EXAMPLE 6

The procedure of Example 2 was exactly repeated except that the diedescribed in Example 5 was used, melt temperature was about 410° F. andthe rate of production of the film was 8.38 pounds/hour/inch of die. Alow level of sharkskin melt fracture was observed during production ofthe film.

The results of Examples 2 to 6 are summarized in Table I.

                  TABLE I                                                         ______________________________________                                        Die                                                                           Ex-  Die                   Rate   Die                                         am-  gap     Die           (lb/hr/in                                                                            temp Melt                                   ple  (mils)  design        of die)                                                                              (°F.)                                                                       Fracture                               ______________________________________                                        2    40      Parallel surfaces                                                                           7.27   400  High level                             3    40      Parallel surfaces                                                                           4.14   380  High level                             4    80      Parallel surfaces                                                                           7.38   390  Low level                              5    80      One side divergent                                                                          7.22   398  None                                   6    80      One side divergent                                                                          8.38   410  Low level                              ______________________________________                                    

The data of Table I show that the final die gap opening is the primarygeometric factor controlling sharkskin melt fracture. Examples 2 and 3in which the sides of the die are parallel and which have a die gap of40 mils produce film with a high level of melt fracture. This high levelof melt fracture occurs in Example 3 with the much lower rate offormation of film. A comparison of a die having one surface of the dielip at a divergent angle from the flow axis of the melt through the die(a die of the present invention, Example 5) with a die having parallelsides (Example 4), with the die gap of each die=80 mils, no meltfracture occurs with the die of the present invention. Even at a higherproduction rate and higher die temperature, the die of the presentinvention (Example 6) produces a low level of melt fracture.

EXAMPLE 7

An ethylene-butene copolymer prepared as in Example 1 and having adensity of 0.924 and a melt index of 2.0 was formed into a film of 1.5mil gauge by blown film extrusion using a 21/2 inch diameter screwextruder as described in Example 2.

The die with the configuration as shown in FIG. 3i was used. Angleα=5.7°, the die gap (dimension k)=100 mils, m=500 mils, and n=60 mils.The melt temperature of the copolymer was about 400° F. Nip roll heightwas approximately 15 ft. Cooling was accomplished with a Venturi typeair ring. Blow up ratio was 2:1. The rate of production of the film was7.0 lbs/hour/inch. Sharkskin melt fracture was determined as in Example2. No melt fracture was observed during production of the film.

EXAMPLE 8

The procedure of Example 7 was exactly repeated except that the angle αof the die was 20° and m=0.110 inches. No melt fracture was observedduring production of the film.

EXAMPLE 9

The procedure of Example 7 was exactly repeated except that the angle ofthe die was 40° and m=0.050 inches. No melt fracture was observed duringproduction of the film.

EXAMPLE 10

The procedure of Example 7 was exactly repeated except that the angle αof the die was 0°, and m=0 inches, i.e., the sides of the die wereparallel. A high level of melt fracture was observed during productionof the film.

These Examples 7 to 10 demonstrate that when using the die of thepresent invention (Examples 7 to 9), no melt fracture was observed evenwhen the angle of divergence was as high as 40° (Example 9). When thedie sides were parallel, a high level of melt fracture was observed fora die gap equivalent to the upstream land separation before thedivergent section.

EXAMPLE 11

An ethylene-butene copolymer prepared as in Example 1 and having adensity of 0.919 and a melt index of 2.0 was formed into a rod using acapillary rheometer.

The rod die was 2.40 cm long with parallel sides and a constant internaldiameter of 0.123 cm. The melt temperature of the copolymer was 180° C.The polymer was extruded at a volumetric flow rate of 0.011 cm³ /sec.

The apparent shear rate γ.sub.α was determined according to thefollowing equation:

    apparent shear rate γ.sub.α =(4Q/πr.sup.3), sec.sup.-1

Q=volumetric flow rate, cm³ /sec

r=internal radius of the die, cm.

The apparent shear rate at the onset of melt fracture was 60 sec⁻¹ (withr in the equation=radius of the die at the exit). Also, the channelshear rate was 60 sec (with r in the equation=radius of the channel.)

EXAMPLE 12

The procedure of Example 11 was exactly repeated except that the rod diewas 2.54 cm. long with entry section 0.337 cm in diameter and die gap of0.126 cm. The die lip was convergent with the same cross section asshown in FIG. 3iii with the angle of convergence=10°.

The melt temperature of the copolymer was 180° C. The polymer wasextruded at a volumetric flow rate of 0.0244 cm³ /sec. The apparentshear rate was determined as in Example II. The apparent shear rate atthe onset of melt fracture was 125 sec⁻¹ (with r in the equation=radiusof the die at the exit). Also, the channel shear rate was 7 sec⁻¹ (withr in the equation=radius of the channel)

The data of Examples 11 and 12 show that at approximately the same exitdiameter, using the die of the present invention (Example 12) abouttwice the flow rate of polymer through the die is possible before theonset of melt fracture as compared to a die, with parallel sides.

What is claimed is:
 1. A process for reducing sharkskin melt fractureduring extrusion of a molten narrow molecular weight distributionethylene polymer into film form under film forming conditions of flowrate and melt temperature which would otherwise produce sharkskin meltfracture, which comprises extruding said polymer through a die having adie gap greater than about 50 mils and wherein at least a portion of onesurface of the die lip and/or die land in contact with the moltenpolymer is at an angle of divergence or convergence relative to the axisof flow of the molten polymer through the die.
 2. A process as in claim1 wherein the die gap is greater than about 50 mils to about 200 mils.3. A process as in claim 1 wherein at least a portion of the surface ofthe die gap and/or die land is at a divergent angle from the axis of theflow of the molten polymer.
 4. A process as in claim 3 wherein thedivergent angle is from 1° to about 45° from the axis of flow of themolten polymer.
 5. A process as in claim 1 wherein at least a portion ofthe surface of the die gap and/or die land is at a convergent angle fromthe axis of flow of the molten polymer.
 6. A process as in claim 5wherein the convergent angle is from about 5° to about 50°.
 7. A processas in claim 1 wherein the ethylene polymer is formed into blown film. 8.A process as in claim 1 wherein the ethylene polymer is a low densityethylene hydrocarbon copolymer.
 9. A process as in claim 8 in which saidcopolymer is a copolymer of ethylene and at least one C₃ to C₆ alphaolefin having a melt index of about ≧0.1 to about ≦20.
 10. A process asin claim 9 in which said copolymer is a copolymer of ≧90 mol percentethylene and ≧10 mol percent of at least one C₃ to C₆ alpha olefin. 11.A process as in claim 10 in which said copolymer has a molecular weightdistribution of about ≧2.2 to ≦3.6 and a total unsaturation content ofabout ≧0.1 to ≦0.3 C=C/1000 C atoms.
 12. A process as in claim 10 inwhich said copolymer has a melt flow ratio of about ≧18 to ≦36 and atotal unsaturation content of about ≧0.1 to ≦0.3 C=C/1000 C atoms.